Homogenous Charge Compression Ignition (HCCI) and spark assisted compression ignition (SACI) are advanced combustion concepts for piston engines that offer significant efficiency and emissions benefits over current technologies. The HCCI combustion process has been studied for over two decades, and has shown promise as a potential technology for automotive engines that can improve on the efficiency and emissions capabilities of current technologies.
In HCCI, a homogeneous mixture of air, fuel and hot exhaust gases is compressed until auto-ignition occurs. Consequently, combustion is not initiated by a spark. Rather, precise conditions are established within a cylinder such that simply by compressing the gases exhibiting the necessary thermodynamic conditions within the cylinder ignition is initiated. HCCI is thus highly dependent upon the in-cylinder temperature and composition of gases (i.e., thermodynamic conditions for the mixture). In order to provide a desired temperature for HCCI ignition, a significant amount of hot exhaust gas from the previous combustion cycle is typically trapped within the cylinder to enable this auto-ignition; however, other methods for initiating HCCI have also been tested, including increasing the compression ratio and heating the intake air.
A significant benefit to incorporating HCCI is that HCCI engines can be run fully unthrottled, significantly reducing pumping losses that are typical in a spark-ignited (SI) engine, thereby boosting the efficiency. Additionally, due to the highly diluted reactant mixture and absence of a flame, peak combustion temperatures are much lower, which reduces NOx emissions significantly.
Operating an engine solely in HCCI, however, is not possible because of engine load and speed limitations. Auto-ignition occurs with very high pressure rise rates leading to the phenomenon of ringing at higher loads which is structurally undesirable for the engine. Hence there is a cap on the maximum power output in HCCI. At the low load end, HCCI mode is harder to maintain because the temperature required to auto-ignite cannot be achieved. HCCI mode is also not possible at lower speeds as the chemical breakdown of species to initiate auto-ignition slows down significantly at lower speeds. This leads to unstable operation or misfire.
SACI has been studied as one approach to smoothing the transition between SI and HCCI, and as an alternative to HCCI. In SACI, a spark is used along with compression. The compression in SACI is typically insufficient to induce spontaneous combustion. Accordingly, the spark controls the timing of the ignition. Due to the lower rates of pressure rise, it is possible to run SACI up to much higher loads than HCCI (>5 bar BMEP), while still deriving some of the same benefits in terms of efficiency (due to unthrottled operation). However, due to the presence of a flame and the high temperatures associated therewith, NOx production is non-negligible—therefore it is necessary to operate SACI at stoichiometric conditions (lambda=1), which allows a three-way catalyst to purge the NOx from the exhaust. Thus, while timing can be precisely controlled, the benefits of HCCI are not fully realized. Further, while SACI allows an extension of the HCCI operating range to higher loads, it is still not sufficient to cover the entire operating range of the engine.
Therefore there exists only a limited operational region for running an engine efficiently and stably in HCCI or SACI mode. Accordingly, attempts have been made to incorporate these modes in an automotive engine by combining them with the conventional SI mode. In these approaches, SI mode is used during cold startup periods and while ramping up the engine through low-speed and low loads. In the region of medium to medium-high loads, the engine can be operated in HCCI or SACI mode, maximizing efficiency and minimizing emissions. The mode can be switched back to SI when the power demand exceeds the upper-load limit of these advanced combustion modes.
Transitioning smoothly from one mode to another however, presents additional challenges. For example, maintaining a desired torque during mode switching can be challenging due to the significant differences between SI and HCCI/SACI operating conditions. Therefore, implementation of HCCI/SACI on a production engine requires advanced control algorithms. The control algorithms are complicated due to the lack of a direct ignition trigger (such as a spark), and the cycle-to-cycle dynamics introduced by the trapped exhaust gas in an HCCI mode. Several modeling and control approaches for steady-state and transient control of HCCI have been presented in the literature.
As is evident from the foregoing discussion, transitions between HCCI/SACI and traditional SI mode are necessary both at the low load/speed as well as the mid-high load/speed end of the operating range. This is shown schematically in FIG. 1. In FIG. 1, the region of engine loads/engine speeds wherein HCCI/SACI mode is advantageous is indicated by area 10. The area 12 identifies the allowable operating region of SI mode. Accordingly, as an engine transitions along a trajectory 14 from a low speed/low load condition to a high speed high load condition, the engine will optimally transition from SI mode to HCCUSACI mode at location 16 and transition from HCCI/SACI mode to SI mode again at location 18. Similarly, as an engine transitions along a trajectory 20 from a high speed/high load condition to a low speed/low load condition, the engine will optimally transition from SI mode to HCCI/SACI mode at location 22 and transition from HCCI/SACI mode to SI mode again at location 24.
Different approaches for switching between SI and advanced combustion modes are known, including single-step switches and transitions that happen more gradually over several cycles. The existing approaches, however, do not incorporate a supervisory control algorithm to determine when switches should be initiated, and when the engine should operate in different combustion modes.
What is needed, therefore, is a control system which determines the most appropriate combustion mode to operate in at any given instant of time, initiates combustion mode switches as necessary, and provides maximal efficiency even during rapid transients and while operating at the boundaries of the different combustion modes.